Spreaders of the type are known from ports and terminals where they are used for transferring shipping containers from ships to road- or railway vehicles, and vice versa. Typically, the spreader forms a part of an arrangement that further comprises cable sheaves by means of which the spreader is supported from a crane that operates and controls the moves of the spreader, and the spreader may be detachably coupled to the arrangement. Among the typical spreader movements are lowering of the spreader for coupling the spreader to one or several shipping containers, lifting and lowering of the spreader and container together, and horizontal shifting of spreader and container from one storage location to another. During these movements, the spreader or the container supported in the spreader may occasionally hit a nearby container or other stationary structure in the vicinity of the spreader, which is a frequent problem as the spreader is usually operated in narrow spaces such as in a ship's stowing space or between container stacks on ships or in container storage yards. Considering load weights in the order of thousands of kilograms and associated accelerations to be handled by the spreader it is readily seen that such occasional collisions may generate considerably high and destroying forces.
The present invention is useful in spreaders that comprise beams which are supported to be moved telescopically in a housing in order to adjust the operative length of the spreader to the length of the subject shipping container. The invention is also applicable to spreaders designed for handling two containers simultaneously, or for the simultaneous handling of four containers, if appropriate. A common feature among these spreaders is that two beams, or two pairs of beams, are associated with a common drive having a pusher acting on the beams, and by which the beams or pairs of beams can be driven for extension or retraction, respectively, in mutually opposite directions in the longitudinal direction of the spreader. In previous solutions, the pusher forms a fixed connection between the beams and a driving means which is effective also for holding the beams in the subject set position. In the event of a collision with a beam end, the shock load applied from the collision is transferred to the driving means via the pusher. If the shock load is heavy enough, damage may result in the driving means, in the pusher, and in other interconnected components.
In hydraulically operated spreaders this problem may at least partly be remedied by means of overload valves, e.g., that open when the system pressure reaches a predetermined, abnormal value. In an electrically operated spreader, where the drive means is an electrical motor controlling the telescopic movements of the beams, a similar remedy is not available in the electromagnetically operated brake that acts on the motor axis and which is typically used for holding the motor and the beams in the subject position.